The principle governing the operation of the read sensor in a magnetic disk storage device is the change of resistivity of certain materials in the presence of a magnetic field (MR or magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of what is termed a bottom spin valve, as seen in FIG. 1, are, starting at the topmost level, free magnetic layer 12, non-magnetic spacer layer 13, magnetically pinned layer 14, and pinning layer 15. One advantage of this type of design is that a capping layer, if made of certain materials, can, in addition to protecting the GMR stack from corrosion, also bring about more specular reflection at the free layer-capping layer interface, thereby increasing the conductance. Isolation of the device from extraneous magnetic fields is achieved by sandwiching it between two magnetic shield layers.
Although the layers enumerated above are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. As first shown by Barkhausen in 1919, magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide operating conditions conducive to single-domain films for the GMR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps. This is most commonly accomplished by giving the structure a permanent longitudinal bias provided by two opposing permanent magnets.
One way to implement this is with an exchange biased magnetic layer as also shown in FIG. 1 Seen there is soft magnetic layer 16 which is permanently magnetized through exchange coupling with antiferromagnetic (AFM) layer 17. To ensure magnetic continuity in the GMR sensor, it is preferred that the top magnetic layer not be touched during processing.
In the case of bottom spin valves, it would, in theory, be ideal if the AFM layer could be placed directly in contact with the free layer, with the sensor region being left uncovered to sense the external media field. This is illustrated in FIG. 2. This structure is, however, difficult to build in practice. This is because the GMR film needs to be capped with a layer such a Ta or Ru for corrosion protection of the GMR film and enhanced specular reflection at the free layer outer interface. Once the capping layers are removed the placement of an antiferromagnet directly on top of the free layer presents a problem as building exchange coupling requires a very clean interface. Exchange biased contiguous junctions have been used with GMR devices however their principle of operation are similar to hard biased contiguous junctions.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,266,218, Carey et al. show a MR with a Bottom Spin Valve and patterned exchange process while Pinarbasi describes a MR with a Bottom SV and buffer layer in U.S. Pat. No. 6,275,362. U.S. Pat. No. 6,310,751 (Guo et al.) shows a pattern exchange for a DSMR, U.S. Pat. No. 6,308,400 (Liao et al.) also discloses a pattern exchange for a DSMR and U.S. Pat. No. 5,637,235 (Kim) discloses a Bottom SV.